'.206 METALLURGY OF IRON AND STEEL.
over 19 per cent. Si02, the rephospliorization need not exceed .015 nor average over .005 per cent.
(3) With slags containing from 10 to 15 per cent. P205 and not over 17 per cent. Si02, the rephosphorization need not exceed .02 nor average over .005 per cent. •
(4) With slags containing from 15 to 20 per cent. P205 and not over 12 per cent. Si02, the rephosphorization need not exceed .02 nor average over .0.1 per cent.
In.using phosphoric stock it is not safe to presuppose the elimination of phosphorus below .04 per cent, until the carbon has been lowered to .08 per cent. Hence to make rail steel it is necessary to eliminate the carbon to that point and then add the required amount of recarburizer, as in the Bessemer process. It is impracticable to use melted spiegel-iron in open-hearth practice, unless there are a great number of furnaces,, because the charges come so irregularly and at such long intervals that a cupola becomes chilled, but it. has been found possible to add finely divided carbon in the ladle, its absorption by the metal being so rapid that the results are quite regular.